This invention relates to microwave resonators or filters, and more particularly, to magnetically tunable filters.
Magnetically tunable filters which use a ferromagnetic material such as yttrium-iron-garnet (YIG) are well known in the art. Generally these filters operate in the gigahertz range and involve multiple conductors on a single ferromagnetic substrate to provide the coupling and filter characteristics of the device. However these structures have several electrical and physical drawbacks when utilized at UHF (225-400 MHz) frequencies.
The electrical limitations include a practical limitation on the number of filter poles obtainable by simple cascading of individual resonators. Most prior art magnetically tuned filters are limited to a two pole configuration. Also the general bandpass characteristics are fairly broad and uneven across the frequency range of the filter. Thus a 10 megahertz bandwidth is considered state-of-the-art in most present magnetically tunable filters operating at UHF frequencies. Moreover, the bandpass characteristics generally change appreciably over the frequency range of the filter, being very narrow at one end and very broad at the other end of the frequency range.
The physical limitations of the present magnetically tuned filters arise from the characteristics of the ferromagnetic material. The ferromagnetic material by its very nature is highly influenced physically by an applied magnetic field. Thus the ferromagnetic material will experience an attractive force toward one of the pole pieces of a magnet. This ferromagnetic material is positioned between the center electrical conductor and the ground plane in usually a microstrip or stripline configuration. Movement of the ferromagnetic material by a magnetic field tends to displace the physical location of the center conductor with respect to the ground plane which in turn causes an uneven change in the electrical characteristics of the filter. Also since the commonly used yttrium-iron-garnet material tends to be brittle like glass, trying to tightly clamp the YIG material in place so as to prevent movement can result in a cracking of the YIG material.
Thus it can be appreciated that a UHF magnetically tuned resonator which is cascadable, which can be formed into narrow band filters with constant bandwidth over the frequency range, and is fairly easy to manufacture is highly desirable.